1. Field of the Invention
The present invention relates to an optical semiconductor device having a distributed Bragg reflector layer; specifically to an optical semiconductor device having a favorable linearity of response and high quantum efficiency.
2. Background Art
A photodiode having a distributed Bragg reflector (DBR) layer between an optical absorption layer and a semiconductor substrate has been proposed. Transmitted light not absorbed in the optical absorption layer is reflected by the DBR layer, and is absorbed again in the optical absorption layer. Thereby, high quantum efficiency is obtained in a photodiode having a DBR layer.
In certain applications, optical signals transmitted through optical fibers are subjected to photoelectric conversion by a photodiode to convert into electrical signals, and are released as they are as electric waves without involving an amplifier of electricity. In this case, the photodiode must not be destroyed even if extremely strong light enters, and must response with favorable linearity. To improve the linearity of response, the heat dissipation of the photodiode must be improved to suppress the temperature elevation in the vicinity of the optical absorption layer. This is because if the temperature of the optical absorption layer is elevated, the transfer rate of electrons and holes generated by light absorption is lowered, the electrons and holes retaining in the optical absorption layer block electric field in the optical absorption layer (space-charge effect), and no current flows.
However, a tertiary mixed crystal semiconductor such as AlInAs and a quaternary mixed crystal semiconductor such as InGaAsP, which are materials for composing the DBR layer, have nearly ten-fold higher thermal resistance than binary mixed crystal semiconductor such as InP and GaAs, which are materials for composing the semiconductor substrate. Therefore, a photodiode having a DBR layer has a problem wherein heat generated in the optical absorption layer is difficult to dissipate in the semiconductor substrate.
Such a problem of the dissipation of the DBR layer is markedly observed in a vertical-cavity surface-emitting laser (VCSEL), which is a light-emitting element sensitive to temperature elevation. As a countermeasure for such a problem, there have been disclosed methods wherein among the two-layer pair of DBR layers, a tertiary mixed crystal semiconductor layer or a quaternary mixed crystal semiconductor layer having a high thermal resistance is thinned, and a layer having a low thermal resistance is thickened (for example, refer to Japanese Patent Application Laid-Open No. 5-283808, Japanese Patent Application Laid-Open No. 2005-354061 and Japanese Patent Application Laid-Open No. 2005-19599).